[S-CG47] Crustal fluids and deformation

[SCG47-P05] Seawater radon concentration in relation to submarine active faults and the seismicity in the Yatsushiro Sea, Japan

*Kuniyo Kawabata¹, Fumiaki Tsunomori³, Yujin Kitamura², Yen-Yu Lin^4,5, Chung-Han Chan^5,4, Kuo-Fong Ma^6,5 (1.Center for General Education, Institute for Comprehensive Education, KAGOSHIMA UNIVERSITY, 2.Graduate School of Science and Engineering, Kagoshima University, 3.Geochemical Research Center, Graduate School of Science, The University of Tokyo, 4.Department of Earth Sciences, National Central University, 5.Earthquake-Disaster & Risk Evaluation and Management (E-DREaM) Center, National Central University, 6.Institute of Earth Sciences, Academia Sinica)

Keywords:Yatsushiro submarine active faults, Radon concentration, Earthquake, Yatsushiro Sea

We investigated the relationship between radon (²²²Rn) concentrations in seawater and crustal activity in the Yatsushiro Sea by studying the submarine fault zone. Yatsushiro submarine faults are located at the southern end of the Futagawa-Hinagu fault zone, which was activated by the 2016 Kumamoto earthquake (Mw 7.0). We analyzed the ²²²Rn concentration in samples of bottom water just above the seafloor, and pore water in sediments, obtained using Multiple and Piston cores of the Hakuho Maru Expedition KH18-3 (July 27-30, 2018).
Typically, ²²²Rn is rarely detected in seawater. However, we observed higher ²²²Rn concentrations around the middle sites in the Yatsushiro Sea. Sediments around these sites are quasi-homogeneous, which primarily consist of sand and mud. ²²²Rn from river water is negligible around the area. For crustal activity, higher-stress field and dense active faults are reported around the sites. Seismic analysis show that the high ²²²Rn concentration regions align with the area seismically most active. Seismicity analysis also revealed high moment release and a low b-value after the 2016 Kumamoto earthquake around the region, indicating increased seismic activity and potential for large earthquakes around the Yatsushiro Sea.
Our results suggest that heightened concentrations of ²²²Rn in seawater can serve as an effective tracer for estimating submarine fault activity. This study highlights the usefulness of ²²²Rn concentrations in the detection of submarine active faults and activity assessment and contributes to the understanding of the potential for future large earthquakes in the Yatsushiro Sea.